half-brothers or sisters. people who share one parent, but not the other. eg, people with the same mother but different fathers.
you could also spell or pronounce it "halvsies", but you'd look like a retard.
Girl1: "I thought you had a brother. How come I never see him anymore?"
Girl2: "My sister and I live here with our mom, but our halfsies live in Texas with their dad."
Guy1: "Megan and Ryan are spending a lot of time together. Are they dating?"
Guy2: "No way, man! They're halfsies!"